The disclosure relates to a method for operating an engine assembly comprising an internal combustion engine having a turbocharger compressor of a turbocharger and having a charge air cooler, which is disposed between the turbocharger compressor and the internal combustion engine in the combustion-air flow direction, and having a throttle valve, which is disposed upstream of the internal combustion engine and downstream of the turbocharger compressor in the combustion-air flow direction.
In order to increase engine performance, modern internal combustion engines are charged with the aid of a turbocharger. The turbocharger compressor compresses ambient air, and so the pressure and the temperature increase correspondingly. The compressed air is subsequently cooled with the aid of a charge air cooler.
A charge air cooler is a heat exchanger, which, in the intake port of a charged internal combustion engine, reduces the temperature of the combustion air supplied to the internal combustion engine. The charge air cooler is installed in the intake port, between the compressor (compressor impeller of a turbocharger) and the intake valve, and dissipates a portion of the heat produced by the compression of the air in the turbocharger. As a result of the cooling, the performance and the efficiency of the internal combustion engine are increased, because, due to the reduction of the temperature of the supplied air, a greater air mass is contained in the same volume. Therefore, proportionally more fuel may be combusted. The charge air cooler therefore increases the potential power output.
Due to the cooling of charge air, the air pressure, and the dew point of the combustion air may increase. Therefore, at high ambient humidity and part load operation, for example, the temperature of the combustion air in the charge air cooler can drop below the dew point and condensate may form within the charge air cooler.
Methods and systems for estimation of an ambient humidity value and for determination of condensate formation (condensate level) in a charge air cooler are known from US 2014/0123963 A1. The ambient humidity value is determined on the basis of a limit value for charge air cooler efficiency and on the basis of a limit value for the speed of a windshield wiper of the front windshield. The humidity value is used for calculating a quantity of the condensate in a charge air cooler and for controlling internal combustion engine systems for reducing condensate formation and for reducing misfiring.
Methods and systems for reducing the corrosion of a charge air cooler and for reducing misfiring due to condensate formation are known from US 2014/0110488 A1. As a reaction to an area of condensate formation in a charge air cooler, a radiator-grill closing system is adjusted, wherein the condensate area is moved to another location in the charge air cooler. The radiator-grill closing orientation can also be controlled in reaction to the operating conditions of the vehicle and to the condensate-forming weather conditions.
The charge air cooler can incur damage during winter, however, if the condensed water freezes in the interior of the charge air cooler, for example. As such, during an acceleration, a large quantity of condensate can enter the combustion chambers of the internal combustion engine, which can result in misfiring. There is a need, therefore, to improve the operational reliability of internal combustion engines charged with a turbocharger at low external temperatures and/or under humid ambient conditions. The inventors herein have identified an approach by which the issues described above may be at least partly addressed. One example method for a vehicle engine comprises: estimating a relative humidity, and reducing a throttle valve differential pressure as a function of the relative humidity, wherein the engine comprises a chargeair cooler disposed downstream of a turbocharger compressor and upstream of an intake throttle valve.
In one example, during vehicle operation, a reduction of the throttle valve position (further closing of throttle valve) of the engine is normally carried out at elevated charge-air pressure in order to increase or optimize the speed of response to sudden requests for acceleration by the driver. According to the disclosure, a relative humidity is determined and, if a limit value for the relative humidity is exceeded, the open position of the throttle valve is increased (further opening of throttle valve) and consequently the charge-air pressure is simultaneously reduced. The reduced charge-air pressure results in a lowering of the dew point of the combustion air (within the charge air cooler) and therefore reduces the tendency for condensate formation in the charge air cooler. Relative humidity is understood to be a measure indicating the ratio of the instantaneous water vapor content with respect to the maximum possible water vapor content for a current temperature and a current pressure, since a given volume of air can absorb only a certain maximum quantity of water vapor as a function of the temperature. At 100% relative humidity, the air is fully saturated with water vapor, whereas, at a relative humidity of over 100%, the excess moisture condenses as condensate or mist. The dew point, which is also referred to as the dew point temperature, is understood to be the temperature below which, (provided the pressure remains unchanged) water vapor may precipitate out of humid air as dew or mist. At the dew point, the relative humidity is 100% or the air is (just) saturated with water vapor.
According to one embodiment, the relative humidity is compared to a threshold humidity value and the open position of the throttle valve may be changed if the relative humidity is greater than the threshold humidity value. In this way, the open position of the throttle valve is increased during critical conditions when condensate formation is expected. This is the case when the humidity is close to or at 100%. For example, the threshold humidity value may be in the range of 90% to 100%.
According to another embodiment, a throttle valve differential pressure may be determined and compared to a threshold differential pressure value and the open position of the throttle valve is changed if the throttle valve differential pressure is greater than the threshold differential pressure value. The threshold differential pressure limit value may be zero or close to zero. In this case, the throttle valve differential pressure may be measured using a pressure sensor, or the throttle valve differential pressure may be estimated by evaluating the throttle valve position. The throttle valve is opened only if a throttle valve differential pressure is present which can be reduced by opening the throttle valve. The reliability of the method is therefore increased.
In order to determine the relative humidity, an ambient temperature value and/or a charge-air temperature value and/or a charge-air pressure and/or an ambient humidity value may be estimated from corresponding sensors. The humidity value may be directly determined by a humidity sensor disposed in the charge air cooler. The relative humidity within the charge air cooler can therefore be precisely determined by capturing and evaluating and, if necessary, combining multiple measured values. This further increases the reliability of the method.
According to a further embodiment, an operating variable of a cooling-air supply device for cooling the charge air cooler may be determined and compared to a threshold operating-variable value, and an open position of a cooling-air supply device may be reduced if the operating variable is less than the threshold operating-variable value. By reducing the open position of the cooling-air supply device, temperature of the charge air cooler may be increased which may hinder further condensate formation in the cooler. In this case, an actuating signal for fully opening the cooling-air supply device can be overwritten. The operating variable may be, as an example, an opening extent of an adjustable active grille shutter system (AGS) coupled in the front end of the vehicle. Cooling air may enter the engine system including the charge air cooler via the adjustable shutters. The operating variable can be, as another example, the air-conditioning system pressure (AC head pressure). In order to operate the air-conditioning system as efficiently as possible, a low air-conditioning system pressure is striven for, which is achieved by means of an open position of the cooling-air supply device that is as large as possible. In the event of a risk of condensation formation, it is ensured that the cooling-air supply device is opened only so far that a sufficient air-conditioning system pressure is ensured. The reduced cooling effect resulting from the reduction of the cooling-air supply therefore counteracts condensate formation.
Further, a charge-air pressure of the internal combustion engine may be reduced if the operating variable is greater than the threshold operating-variable value. Therefore, the dew point may be lowered by reducing the charge-air pressure, which counteracts condensate formation, especially if it is not possible to lower the dew point by any other measures. The performance of the internal combustion engine may be affected only during conditions when there is reduction in the charge-air pressure.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.